The regular and sufficiently accurate and careful performance of measurement and other quality-control measures for patient-care equipment, is the foundation of reliable radiotherapy. Together with changes in the conditions and demands of this work, the development of external radiotherapy methods, accelerators, and equipment relating to the radiotherapy chain has brought new challenges for those working with radiotherapy.
It is difficult for hospital physicists to carry out their duties within the constraints of working time and existing personnel resources. The complexity of the measuring devices and the preparations required by measurements mean that making quality control measurements of patient-care equipment using present equipment is also time consuming. Making measurements takes up the entire working time of hospital physicists, leaving them no time to carry out any other tasks. In addition, the measurements specified in a quality-control program for radiotherapy equipment cannot be made during normal working time, as the equipment is being used for therapy at that time.
Besides changing demands, increased pressure to develop measuring equipment that would accelerate and simplify the tasks of hospital physicists has also arisen from the development of data management and database systems.
One equipment solution according to the state of the art, presently used for quality-control measurements of patient-care equipment, is represented by the so-called water phantom. A water phantom comprises a water-filled plexiglass box, inside which ionization detectors measuring the intensity of the radiation field on an X-Y plane are moved. The ionization detectors, of which there are typically 1–24, are arranged in a comb shape.
The planar movement of the detectors can be limited to either the X or the Y direction. The plane measurement is repeated at different depths in the Z direction. Due to the small number of detectors, the position measurement is accurate to only the order of a few centimeters. Any increase in the number of detectors will significantly raise the price of the device and the amount of complex electronics required.
A measurement made using the detector in question may take up to several hours, during which time the ambient temperature, among other factors, can vary, simultaneously altering the gas pressure in the ionization chamber of the detectors. It is difficult to compensate later for the measurement error that this creates and which in any event reduces the reliability of the measurement.
In addition to the above, due to the measurement principle of sweeping the radiation field of the detectors, the water phantom according to the state of the art cannot measure the intensity distribution of a field independently of time. Practically all new accelerators are time-dependent, so-called dynamic-field accelerators, making it extremely desirable to also be able to determine the intensity distribution of the field.
A second device representing the state of the art is a plane detector comprising typically less than 10 ionization chambers, which is used in high-speed quality assurance measurements to check the stability, evenness, and symmetry of a radiation field. The position resolution ability of these devices is poor, being several centimeters, and they are unable to measure small variations in position in a dynamic field.
On the basis of the state of the art, it can be further asserted that even the latest detector models have considerably lagged far behind the development of accelerators and of other measuring and analysis devices. The fact that known devices are out of date can also be seen in their quite simple user interfaces.
Yet a third device impinging on the state of the art is disclosed in U.S. Pat. No. 4,485,307. The device is intended for radioisotope diagnostics within an organ. In it, the XY-plane detectors set in a gas plenum are formed from two cathode layers. The cathode layer is formed of wires running in one direction and set at equal intervals to each other. The orientation of the wires of the layers is arranged such that the XY position of radiation can be determined on the basis of them. However, apparatus of this type has only a poor ability to determine in real time the shape of the radiation fields of modern high-energy radiotherapy devices creating dynamic fields. Further, radioisotope diagnostics have an operating environment that is, in terms of the energetics and intensity of the radiation field for instance, of a totally different order of magnitude to that of patient-care equipment, thus excluding the use of the apparatus disclosed in the publication in, for example, an accelerator environment.